Arterial baroreceptor properties and their relationship to baroreflexes will be examined in this proposal. Properties of the receptor membrane, its axon and the wall of the artery in which the receptors lie, determine the pattern of activity which is sent to the central nervous system. Analysis of receptor mechanisms has been limited by anatomical problems of smooth muscle or the receptor membrane or axon are responsible for modifying discharge under a variety of conditions. An isolated preparation consisting of the adventitial layer of the aortic arch with depressor nerve intact and the smooth muscle removed has been developed. It will be used to assess such phenomena as post-excitatory depression, resetting and adaptation, as well as the ation of the various substances such as catecholamines, sodium, calcium and potassium. Information will be obtained about the mechanism and sites of action of the mdoulators of baroreceptor activityh and about operation of the afferent limb of the baroreceptor reflexes. Modulation of baroreceptor reflexes. Modulation of baroreceptor discharge has important implications for the manner in which the blood pressure, heart rate and renal function are regulated. The second portion of the proposal will eealuat the reflex effecs of the phenomenon "acute or rapid resettling" which has been described for aortic baroreceptors. In rapid resettling, the threshold for baroreceptor discharge is dependent upon the prevailing pressure. When pressure is increased, the threshold resets to a higher value within fifteen minutes. This differs from rapid adaptation of barorceptors which occurs within second and from the long term resetting of hypertension and has important implications in normal blood pressure control. The dependence of threshold, gain and saturation of the baroreceptor reflex on prevailing pressure is determined in the ct using a perfused sinus as the input for open loop reflex experiments. Preliminary experiments indicate that the resetting process is not confined to the baroreceptors but also occurs in the central nervous system pathways. Experiments will be designed to assess the relative contributions of the baroreceptors and central nervous system components to resetting of the baroreceptor reflexes.